linux-kernel - Re: [RFC][PATCH] create workqueue threads only when needed

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Date:	Mon, 2 Feb 2009 12:32:36 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH] create workqueue threads only when needed

On Mon, Feb 02, 2009 at 09:42:45AM +0100, Stefan Richter wrote:
> Benjamin Herrenschmidt wrote:
> > On Mon, 2009-02-02 at 03:24 +0100, Frederic Weisbecker wrote:
> >> On Mon, Feb 02, 2009 at 08:37:41AM +1100, Benjamin Herrenschmidt wrote:
> >> > 
> >> > > I don't know, most of those I've looked on are not documented about the reason
> >> > > for a private workqueue. I guess most of them can use the usual kevent.
> 
> I rather suspect that the majority of private workqueues are there for
> good reasons.
> 
> >> > The main problem with kevent is that it gets clogged up.
> >> 
> >> I don't think so. Here is a snapshot of the workqueue tracer in my
> >> box currently:
> > 
> > That's not quite what I meant ... 
> > 
> > The main problem with keventd I'd say is that it's used in all sort of
> > exeptional code path (ie, driver reset path, error handling, etc...) for
> > things that will msleep happily for tenth milliseconds, that sort of
> > thing.
> > 
> > IE. It will be pretty responsive -in general- but can suffer form
> > horrible latencies every now and then.
> 
> Actually it /should/ be the other way around:
> 
> The shared workqueue should only be used for work that sleeps only
> briefly (perhaps with the exception of very unlikely longer sleeps e.g.
> for allocations that cause paging).
> 
> Work which /may/ sleep longer, for example performs SCSI transactions,
> needs to go into a private workqueue or other kind of context.


Right. But most of the time, these workqueues receive few events.
That's why async looks a good alternative for such cases. The threads
from async core which perform the jobs are created and destroyed on the fly,
depending on the number of jobs queued.

 
> OTOH you are right too; work which must not be deferred too long by work
> from another uncooperative/ unfair subsystem is probably also better off
> in an own workqueue...



Or callsites which use kevent and may sleep for too long could be identified
and fixed...


> Stefan Richter
> -=====-==--= --=- ---=-
> http://arcgraph.de/sr/

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